RFC 5640

Load-Balancing for Mesh Softwires

Network Working Group C. Filsfils
Request for Comments: 5640 P. Mohapatra
Category: Standards Track C. Pignataro
Cisco Systems
August 2009 Load-Balancing for Mesh Softwires
Abstract
Payloads transported over a Softwire mesh service (as defined by BGP
Encapsulation Subsequent Address Family Identifier (SAFI) information
exchange) often carry a number of identifiable, distinct flows. It
can, in some circumstances, be desirable to distribute these flows
over the equal cost multiple paths (ECMPs) that exist in the packet
switched network. Currently, the payload of a packet entering the
Softwire can only be interpreted by the ingress and egress routers.
Thus, the load-balancing decision of a core router is only based on
the encapsulating header, presenting much less entropy than available
in the payload or the encapsulated header since the Softwire
encapsulation acts in a tunneling fashion. This document describes a
method for achieving comparable load-balancing efficiency in a
network carrying Softwire mesh service over Layer Two Tunneling
Protocol - Version 3 (L2TPv3) over IP or Generic Routing
Encapsulation (GRE) encapsulation to what would be achieved without
such encapsulation.
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (c) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents in effect on the date of
publication of this document (http://trustee.ietf.org/license-info).
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document.

2. Load-Balancing Block sub-TLV
This document defines a new sub-TLV for use with the Tunnel
Encapsulation Attribute defined in [RFC5512]. The new sub-TLV is
referred to as the "Load-Balancing Block sub-TLV" and MAY be included
in any Encapsulation SAFI UPDATE message where load-balancing is
desired.
The sub-TLV type of the Load-Balancing Block sub-TLV is 5. The sub-
TLV length is 2 octets. The value represents the length of the block
in bits and MUST NOT exceed the size of the load-balancing field.
This format is very similar to the variable-length subnet masking
(VLSM) used in IP addresses to allow arbitrary length prefixes. The
block is determined by extracting the initial sequence of 'block
size' bits from the load-balancing field.
If a load-balancing field is not signaled (e.g., if the encapsulation
sub-TLV is not included in an advertisement as in the case of GRE
without a Key), then the Load-Balancing Block sub-TLV MUST NOT be
included.
The smaller the value field of the Load-Balancing Block sub-TLV, the
larger the space for per-flow identification, and hence the better
entropy for potential load-balancing in the core, as well as, the
lower the polarization when mapping flows to ECMP paths. However,
reducing the load-balancing block size consumes more L2TPv3 Session
IDs or GRE Keys, resulting in potentially less numbers of supported
services. A typical deployment would need to arbitrate between this
trade-off.
As an example, assume that there is a Softwire set up between R1 and
R3 with L2TPv3-over-IP tunnel type. Assume that R3 encodes the
Session ID with value 0x1234ABCD in the encapsulation sub-TLV. It
also includes the Load-Balancing Block sub-TLV and encodes the value
24. This should be interpreted as follows:
o If an ingress router does not understand the Load-Balancing Block
sub-TLV, it continues to use the Session ID 0x1234ABCD and
encapsulates all packets with that Session ID.
o If an ingress router understands the Load-Balancing Block sub-TLV,
it picks the first 24 bits out of the Session ID (0x1234AB) to be
used as the block and fills in the lower-order 8 bits with a per-
flow identifier (e.g., it can be determined based on the inner
packet's source, destination addresses, and TCP/UDP ports). This
selection preserves the per-flow ordering of packets.

This requirement and solution applies equally to GRE where the Key
plays the same role as the Session ID in L2TPv3.
Needless to say, if an egress router does not support the Load-
Balancing Block sub-TLV, the Softwire continues to operate with a
single load-balancing field with which all ingress routers
encapsulate.
2.1. Applicability to Tunnel Types
The Load-Balancing Block sub-TLV is applicable to tunnel types that
define a load-balancing field. This document defines load-balancing
fields for tunnel types 1 (L2TPv3 over IP) and 2 (GRE) as follows:
o L2TPv3 over IP - Session ID. Special care needs to be taken to
always create a non-zero Session ID. When an egress router
includes a Load-Balancing Block sub-TLV, it MUST encode the
Session ID field of the encapsulation sub-TLV in a way that
ensures that the most significant bits of the Session ID, after
extracting the block, are non-zero.
o GRE - GRE Key
This document does not define a load-balancing field for the IP-in-IP
tunnel type (tunnel types 7). Future tunnel types that desire to use
the Load-Balancing Block sub-TLV MUST define a load-balancing field
that is part of the encapsulating header.
2.2. Encapsulation Considerations
Fields included in the encapsulation header besides the load-
balancing field are not affected by the Load-Balancing Block sub-TLV.
All other encapsulation fields are shared between variations of the
load-balancing field. For example, for the L2TPv3-over-IP tunnel
type, if the optional cookie is included in the encapsulation sub-TLV
by the egress router during Softwire signaling, it applies to all the
"Session ID" values derived at the ingress router after applying the
load-balancing block as described in this document.
3. IANA Considerations
IANA has assigned the value 5 for the Load-Balancing Block sub-TLV,
in the BGP Tunnel Encapsulation Attribute Sub-TLVs registry (number
space created as part of the publication of [RFC5512]):
Sub-TLV name Value
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Load-Balancing Block 5